Is it normal that my girlfriend's daughter tried to arouse me

My girlfriend and I were laying down on her bed. She had her head on my chest with her eyes closed. Her 12-year old daughter (very tall 5' 7"), came in and laid down next to her mom then through her arm over her. Her hand landed on my penis. I thought nothing of it at first. After a minute, she started lightly moving her fingers up and down my penis. I was getting hard. After a couple of minutes, my girlfriend got up to go to the bathroom. Her daughter reached over and was doing it again. I became so hard. Later, when I thought about her being young, I was like that's unbelievable that she would do that at her age.